he micro-hardness test method is performed with a tester to force a diamond Indentor of specific geometry (Vickers or Knoop) with loads up to 1000 grams into a specimen, and the Indentor impression is measured using a precision microscope attached to the machine. The primary applications are:
- Measuring the hardness of precision components and product forms like foil or wire that are too thin or too small to be measured by bulk test methods.
- Monitoring carburizing, nitriding, or other surface modification operations by hardness traverses taken on cross sections. (Figure 1)
- Measuring the hardness of individual micro-constituents. (Figure 2)
- Measuring at the edge of a test piece to detect surface conditions such as decarburization.
- Measuring the hardness of surface layers such as plated or bonded layers. (Figure 3)
Why do some people use Vickers indenters while others use Knoop?
The Vickers hardness test follows the Brinell principle in that an Indentor of specific shape is pressed into the test material, the load is removed, the diagonal of the resulting indentation are measured, and the hardness number is calculated by dividing the load by the surface area of the indentation. The Indentor is made of diamond and is in the form of a square based pyramid with an angle of 136° between faces. The facets are highly polished, free from surface imperfections, and the point is sharp. The depth of indentation is about one-seventh of the average diagonal length.
The methodology of the Knoop hardness test is similar to the Vickers test with the exception of the Indentor shape. A standard Knoop Indentor is a rhombic-based pyramid which produces an indentation that has an ideal ration between long and short diagonals of approximately 7 to 1. The depth of indentation is about one-thirtieth of the length of the long diagonal.
- Because the Vickers Indentor penetrates deeper into the specimen than the Knoop Indentor at the same load, the Vickers test is less sensitive to surface conditions.
- A Knoop Indentor is often selected for testing surface defects due to the limited penetration depth.
- Both of the diagonal lengths of the Vickers indentation are measured and averaged. This results in a value that many operators have more confidence in.
- The shape of the Knoop Indentor makes it desirable for placement in coatings and elongated micro-constituents. In addition, the shorter diagonal enables the operator to place the indents closer together to detect rapid changes in hardness observed in some case hardened components.
- Vickers test results vary little with the test load (except below 100gf), which increases the relevance of conversion to other test scales, in comparison to Knoop test results.
- Report the hardness value, the type of Indentor used, load force applied, dwell time, magnification used for measurement, and any unusual conditions encountered during the test.
Reporting of a hardness value is typically abbreviated using the following notation: 400 HK200
The hardness value is followed by HK for a Knoop measurement or HV for a Vickers measurement while the load is reported in grams-force by subscript notation according to ASTM E 384. Equipment for micro-hardness testing usually consists of a testing machine that supports the specimen and permits the Indentor and specimen to be brought into contact gradually and smoothly under a predetermined load. The design of the machine should be such that no rocking or lateral movement of the Indentor or specimen is permitted while the load is being applied or removed. A precision measuring microscope is mounted on the machine in such a manner that the indentation may be readily located in the filed of view.
Several types of micro-hardness testers currently are available. Most operate via the direct application of load by dead weight, or by weights and a lever. The various testers vary mainly in load range. Any can accommodate either the Knoop or the Vickers indenters. Some models are console and others are bench-mounted types.
The micro-hardness tester shown in Figure 8 has a load range of 10 to 1,000 grams. Loads are applied by dead weight. The microscope is furnished with three objective lenses 10x, 50x, 100x, and a 10x eyepiece. Sources of tester error include inaccuracy in loading, vibration, rate of load application, duration of contact period, and impact. To limit the shock that can occur when the operator removes the load, an automatic test cycle is built into the micro-hardness tester. With this automatic test cycle, the load is applied at a constant rate, maintained in the work for 10 -15 seconds and smoothly removed. Thus, the operator does not need to touch the tester while the load is being applied and removed. The design of micro-hardness testers will vary from one type to another, but it is essential to remove the applied load without touching the tester if sharp indentations are to be obtained.
Micro-hardness testers are designed to control the operating parameters needed to meet the test specifications. However, to obtain accurate and repeatable tests, the correct test conditions must be set. Choose the load appropriated for the material you are testing and select the dwell time for the specific test according to ASTM or other relevant standards. When all the parameters are set, indirect verification can be accomplished. Place five indentations in a calibrated test block and compare the measured results with the values found on the certificate accompanying the block. Guidelines for machine repeatability are present in ASTM E 384 which takes into account the load applied and the hardness of the calibrated test block. The operator should also be instructed as to the correct use of optical measuring devices. In most testers, there are at least two objective lenses, a low power objective (10x) for location of the test area and a high power objective (40x/50x)) for measuring the indentations. To obtain the maximum accuracy correct positioning of the indentation relative to the eyepiece filar lines is important. One of the heaviest uses of micro-hardness testing is the determination of effective case depth. Effective case depth is defined as the perpendicular distance from the surface of a hardened case to the furthest point where a specified level of hardness (HRC50). This mechanical test method, of applying a series of indentations at known locations, provides greater consistency from operator to operator, and greater precision, than a visual examination of the case depth.
The main application of effective case depth measurements in the heat treatment industry is for Automotive & Truck transmission parts, shafts, and crankshafts. These components require surface treatments to get the required surface hardness and case depth, for their design applications. Because the depth of hardness is often less than a millimeter, several indents will need to be accurately placed to pick up the hardness trend, microhardness testing with a Vickers or Knoop Indentor is ideally suited to this application.
The Automatic Micro-hardness testing system, will allow for repeated positioning of an exact series of indentations and the subsequent reading of the indentation diagonals, with the same pattern of indents repeated on each individual micro-sections, repeated several times on a particular specimen (i.e., tip, flank and root for a gear tooth). Added to this is the need to transcribe the measurements into a data list, graph the results against the distance traversed, and draw the intercept according to the specified hardness limit, the Automatic Optical Micro-hardness testing system shown here and manufactured by Mitutoyo, is a great tool for the operators.
The AAV-500 series reduces individual differences in impression dimension measurement in the microhardness test and Vickers hardness test by adopting special image analysis technologies. In addition, improved precision and high speed have been realized with a detecting time of 0.3 seconds.
A micro-hardness test based on the Rockwell technique has currently been introduced. This tester has a minor load of 100 grams and a full load of 1000 grams. With this instrument no optical measurement is necessary. Depth of penetration can be measured and displayed in equivalent Rockwell C numbers in less than six seconds.
The MT-90, a micro-hardness tester based on the Rockwell Principle (Depth of Penetration Measurement) shown with a traversing stage and computer to automatically traverse to a created program, and digitally read-out the hardness number in HRC. It can also provide Effective Case Depth Charting data.